Burmese amber fossils bridge the gap in the Cretaceous record of polypod ferns

Cretaceous fossils of derived leptosporangiate ferns (Polypodiales) are exceedingly rare yet they are needed to confirm the hypothesis of a Cretaceous diversification of polypod ferns as predicted by DNA-based divergence time estimates. Here we show that diverse polypod ferns existed in the mid-Cretaceous woodlands of Myanmar. We describe isolated sporangia with a vertical, broken annulus ring containing a differentiated stomium as well as distinguished pinnae fragments. Krameropteris resinatus gen. et sp. nov. is described based on a pinnae-fragment with free branched veins, exindusiate sori and polypod sporangia with trilete spores, and assigned to the early diverging polypod fern lineage Dennstaedtiaceae. Integration of the new fossil evidence in a phylogenetic framework provides support to the hypothesis of a Cretaceous diversification of polypod ferns.